Recent studies have revealed a complex biological role for cell membrane phospholipids, which were previously believed to have only a structural function. Following cell activation, membrane phospholipids may be metabolized to eicosanoids and lysophospholipids, which are important regulators of cellular function and behavior. Lysophospholipids include compounds such as lysophosphatidic acid (“LPA”), sphingosine-1-phosphate (“S1P”), lysophosphatidylcholine and sphingosylphosphorylcholine and are important second messengers that can activate particular cell surface transmembrane G-protein coupled receptors known as endothelial gene differentiation (“Edg”) receptors.
Two quite distinct subfamilies of GPCRs bind LPA and S1P specifically and transduce diverse cellular signals by associating with one or more G proteins. Based on amino acid sequence identities, S1P1 (Edg 1), S1P3 (Edg 3), S1P2 (Edg 5), and S1P5 (Edg 8) belong to one structural cluster and LPA1 (Edg 2), LPA2 (Edg 4) and LPA3 (Edg 7) are members of a second structural cluster (Goetzl, E. J., and Lynch, K. R. 2000, Ann. N.Y. Acad. Sci. 905:1–357). Members of both subfamilies range in size from 351 to 400 amino acids, and are encoded by chromosomes 1, 9 or 19. The amino acid sequence of S1P4 (Edg 6) lies between those of the two major clusters by amino acid sequence identity (Graler et al., 1998, Genomics 53:164–169). Edg-6, a novel G-protein-coupled receptor related to receptors for bioactive lysophospholipids, is specifically expressed in lymphoid tissue (Graler et al., 1998, Genomics 53:164–169). Currently, there are three known Edg receptors specifically activated by LPA (LPA1 or Edg 2, LPA2 or Edg 4 and LPA3 or Edg 7) and five known S1P receptors specifically activated by S1P (S1P1 or Edg 1, S1P2 or Edg 5, S1P3 or Edg 3, S1P4 or Edg 6, and S1P5 or Edg 8).
Edg-1 (human Edg-1, GenBank Accession No. AF233365), Edg-3 (human Edg-3, GenBank Accession No. X83864), Edg-5 (human Edg-5, GenBank Accession No. AF034780), Edg-6 (human Edg-6, GenBank Accession No. AJ000479) and Edg-8 (human Edg-8, GenBank Accession No. AF317676) receptors are activated by S1P, while LPA activates Edg-2 (human Edg-2, GenBank Accession No., U78192), Edg-4 (human Edg-4, GenBank Accession Nos. AF233092 or AF011466) and Edg-7 (human Edg-7, GenBank Accession No. AF127138) receptors. Although, all three LPA receptors (i.e., Edg-2, Edg-4 and Edg-7) bind LPA, compounds, which discriminate between these receptors have been identified (Im et al., 2000, Mol. Pharmacol. 57 (4):753–759). Further, Edg 2, Edg-4 and Edg-7 appear to exhibit significant pharmacological differences (Bandoh et al., 2000, FEBS Lett. 478:159–165).
Importantly, Edg receptors are believed to mediate critical cellular events such as cell proliferation and cell migration, which makes these receptors attractive therapeutic targets. However, currently known compounds, which bind to LPA, are almost exclusively phospholipids (e.g., LPA and S1P, analogs of LPA and S1P, dioctyl glycerol, etc). Most of these phospholipids compounds fail to effectively discriminate between different Edg receptors and have poor physicochemical properties, which limits their potential use as pharmaceutical agents. Thus, there exists a need for compounds, which are not phospholipids that bind or otherwise regulate Edg receptors and can also selectively bind to a specific Edg receptor.